1. Field of the Invention
The present invention relates to improvement in a wire-cut electric discharge machine which employs a servo feed method that controls the relative feed rate of a workpiece and a wire electrode so that a mean machining voltage is held substantially constant. More particularly, the invention pertains to a wire-cut electric discharge machine which is adapted to change electrical cutting conditions in accordance with the thickness of a workpiece for efficiently electric discharge machining the workpiece.
2. Description of the Prior Art
In the case of cutting a workpiece through the use of a wire-cut electric discharge machine of the type moving the workpiece and a wire electrode relative to each other and cutting the workpiece by a discharge into a desired shape, efficient cutting can be achieved by making electrical cutting conditions (a no-load voltage, a peak current, ON time, etc.) large in proportion to the thickness of the workpiece.
On account of this, there has also been proposed a wire-cut electric discharge machine adapted to switch the electrical cutting conditions in accordance with the thickness of the workpiece. But, the conventional machine of this kind possesses the following defect. The prior art machine adopts a servo feed to controls the relative feed rate of the workpiece and the wire electrode so that the mean cutting voltage may be constant. This machine determines the thickness of the workpiece on the basis of the feed rate (which decreases with an increase in the thickness of the workpiece) and switches the electrical cutting conditions on the basis of the result of the thickness determination. Accordingly, the reduction in a feed rate due to an arc discharge which does not machine the workpiece but only makes the workpiece surface rough, is regarded to have resulted from an increase in the thickness of the workpiece. Accordingly the electrical cutting conditions are erroneously made large, resulting in further degradation of the state of discharge.